1. Field of the Invention
This invention relates to a radar signal processor having a coherent integration (also called predetection integration) function and a pulse Doppler radar system using the processor.
2. Description of the Related Art
FIGS. 11 and 12 show the configuration of a radar system disclosed in Japanese Utility Model Publication No.Hei 4-5033. The radar system is a pulse radar system which transmits a pulse signal and receives its echo for searching for surrounding targets; it is also a Doppler radar system which can detect a Doppler frequency component contained in the received echo for detecting moving targets. Such a configuration is called a pulse Doppler radar system.
As shown in FIG. 11, an antenna 1 is shared by transmitting and receiving circuits. A transmitting pulse generator 3 generates a transmitting pulse in response to a control signal, such as a signal indicating a pulse repetition frequency or pulse width, supplied from a reference generator 5, and supplies a radio frequency signal of a predetermined frequency modulated by the transmitting pulse via a diplexer 2 to the antenna 1. When receiving the signal from the transmit pulse generator 3, the antenna 1 transmits it to surrounding space, hereinafter referred to as search space, as a radio wave. If a target exists in the search space, the radio wave transmitted through the antenna 1 is reflected by the target. The echo is received by the antenna 1 and is supplied via the diplexer 2 to a frequency converter 4, which then converts the input echo into a predetermined intermediate frequency and further amplifies the resultant intermediate frequency signal.
The intermediate frequency signal amplified by the frequency converter 4 is phase-detected by an in-phase synchronous detector 6I and a quadrature-phase synchronous detector 6Q. The reference phase required for the quadrature-phase detection is given as a reference signal from the reference generator 5. That is, the in-phase synchronous detector 6I detects the intermediate frequency signal in synchronization with the phase of the reference signal (reference phase) and the quadrature-phase synchronous detector 6Q detects the intermediate frequency signal in synchronization with the phase orthogonal to the reference phase. The in-phase component I provided from the in-phase synchronous detector 6I by such an operation is converted from analog into digital form by an A/D converter 7I; the quadrature-phase component Q provided from the quadrature-phase synchronous detector 6Q is converted from analog into digital form by an A/D converter 7Q. Hereinafter, the in-phase component I and quadrature-phase component Q are collectively called a complex signal.
As methods of improving the signal to noise power ratio (SN) of the signal thus obtained, for example, coherent integration, incoherent integration, also called postdetection integration, CFAR (constant false alarm rate) processing, and the like are available. SN improvement sections 9-1, 9-2, . . . 9-L shown in FIG. 11 are circuitry for executing such processing. L SN improvement sections 9-1, 9-2, . . . 9-L (L: A natural number of 2 or greater, for example, 100) are provided as shown in FIG. 11, each of which takes a predetermined range domain. For example, the SN improvement section 9-1 takes a range domain R(1), the SN improvement section 9-2 takes a range domain R(2), . . . the SN improvement section 9-L takes a range domain R(L). The range mentioned here corresponds to the time from transmission of a radio wave to reception of the echo thereof and the range domain means a domain to which one reception point of time, namely, a point at one range from the transmission point belongs. Allocation of the range domains to the SN improvement sections is fixed.
A range divide circuit 8 divides a complex signal into L parts according to the range, namely, components corresponding to range domains in such a manner that it supplies the part of the complex signal in the range domain R(1) taken by the SN improvement section 9-1 to the same, the part in the range domain R(2) to the SN improvement section 9-2, . . . the part in the range domain R(L) to the SN improvement section 9-L. Thus, the range divide circuit 8 inputs transmitting pulses from the transmitting pulse generator 3 to know the transmission timings. The SN improvement sections 9-1, 9-2, . . . 9-L perform processing (described below) for the supplied complex signal parts, and supply the resultant signals to a display 10, which then converts the signals into a radar image for display on a screen thereof. For example, it displays a moving target.
The SN improvement section 9-i (i=1, 2, . . . L) comprises a coherent integrator 11, square detectors 12-1, 12-2, . . . 12N, and a CFAR detector 13, as shown in FIG. 12. The coherent integrator 11 coherently integrates the parts of the divided complex signal supplied from the range dividing circuit 8 for improving the SN. Generally, more than one echo (echo pulse) is obtained from a single target. Assuming that the number of echo pulses obtained from a single target is N (N: A natural number of 2 or greater), the SN can be improved N times by performing conventionally known coherent integration. This means that the target signal to noise power ratio containing white noise generated by the frequency converter 4, the in-phase synchronous detector 6I, the quadrature-phase synchronous detector 6Q, etc., can be improved. Since one range domain R(1) is allocated to the SN improvement section 9-i as described above, the coherent integration performed by the coherent integrator 11 enables a target positioned in the range domain R(1) to be detected at SN as many times as the number of coherent integration points, namely, N times that in the case where no coherent integration is performed. The coherent integrator 11 discriminates the coherent integration result to N Doppler frequency components for output.
Each Doppler frequency component output from the coherent integrator 11 (represented as complex signal in the figure) is input to any of the square detectors 12-1, 12-2, . . . 12-N. Each square detector makes square detection of the input complex signal and outputs the result to the CFAR detector 13, which then compares the square detection output with a threshold to remove noise generated, for example, by clutter. The threshold is defined in response to the square detection output value.
Assume that one of the N square detection outputs arranged along the Doppler frequency axis is observed. The threshold used for comparison with the square detection output value corresponding to the observed Doppler frequency component, namely, object Doppler frequency component, is the average value or weighted average value of a total of M (M: A natural number of 2 or greater) square detection output values corresponding to a total of M Doppler frequency components before and after the object Doppler frequency component, namely, reference Doppler frequency components. The CFAR detector 13 compares each square detection output value with the threshold found as described above. For example, the square detection output values equal to or greater than the threshold value are converted into "1"; other values, "0."
The rate at which noise such as clutter is erroneously recognized as a target, the false alarm rate, can be made constant by performing such threshold comparison and conversion processing. Since noise generated by clutter, etc. is wide compared with a target, the noise level is reflected in the average value or weighted average value of the square detection output values and a change in the noise level is also reflected in the average value or weighted average value of the square detection output values. Therefore, the average value or weighted average value of the square detection output values of the M reference Doppler frequency components before and after the object Doppler frequency component is found for use as the threshold, whereby the false alarm rate can be made constant regardless of the occurrence of noise or its tendency. The conversion result is supplied to the display 10.
Setting the number of coherent integration points, N, introduces a problem in such a conventional radar system. If the number of coherent integration points, N, is made large, generally the number of echo pulses that can be integrated increases, so that improved performance of the SN of the coherent integrator 11 results. In contrast, if the number of coherent integration points, N, is made large, it will take time for coherent integration; resultantly, the time from reception of echo to target detection, namely, the observation time is prolonged. If the observation time is prolonged, rapid detection of the target is made impossible, delaying taking steps for a near range target, such as a moving target existing in a near range, for which steps should be taken as promptly as possible after detection. If the number of coherent integration points, N, is made small, the observation time can be shortened to take instant steps for a near range target, but the SN improvement effect by the coherent integration cannot be expected to be very much.
Hitherto, the number of coherent integration points, N, has been set to a comparatively large value considering such a disadvantage. That is, the number of coherent integration points, N, has been set in response to the maximum range (maximum detection range) in which the minimum target having a predetermined size can be detected, and the problem of immediate response such as early detection of near range targets remains unsolved.